The present invention relates to improvements in the electron lens system of a transmission-type electron microscope.
In recent years, it has become increasingly important for transmission-type electron microscopes to permit analysis of the physical properties of microscopic regions in a microscope image derived from a sample, as well as observation of the microscope image. In order to make possible such an analysis, it is necessary that a sufficiently large current, or illuminance, of electron beam is given to the microscopic region on the sample that is irradiated with the electron beam focused to a size of several nanometers to tens of nanometers on the sample. Thus, a lens exhibiting only a small spherical aberration is required to be used for the focusing lens at the final stage which contributes to the focusing of the electron beam. If the excitation to the objective lens were varied between the observation of a micrograph and the analysis of a microscopic region, it would be difficult to make their fields of view correspond to each other. Hence, it is also necessary that the objective lens is excited constantly, whether the observation or the analysis is made. In order to observe specimens while satisfying these requirements, the specimens have been conventionally placed in a so-called condense objective position within the magnetic field that is produced by the strongly energized objective lens.
FIG. 1 is a schematic diagram showing the electron beam path in a conventional (prior art) electron microscope. The optical analogs of electron lens are used schematically in the drawings. A specimen 1 is placed in the condense objective position defined by an objective lens 2, 3, and which is put in transmission electron microscopy (TEM) mode. FIG. 2 is a diagram similar to FIG. 1 except that the microscope is put in analysis mode. In these two figures, a filmy specimen 1 to be examined is placed in the magnetic field set up by the objective lens, which can be considered to be composed of a front objective lens 2 disposed above the specimen 1 and a rear objective lens 3 disposed below the specimen 1. These lenses can either project the infinitely remote point above the specimen onto the specimen or converge the electron beam transmitted through the specimen at the infinitely remote point below the specimen without varying the excitation to the objective lenses. The electron beam 4 that is to be directed to the specimen 1 is emitted by an electron gun 5 in a divergent manner. The beam 4 is focused by first, second, third focusing lenses 6a, 6b, 7, respectively, and the front objective lens 2 in turn. The electron beam transmitted through the specimen 1 passes through an objective lens aperture plate 8 and is directed into an imaging lens system (not shown) containing intermediate lenses.
In the TEM mode shown in FIG. 1, the electron beam 4 is caused by the focusing lens 7 to produce a crossover image of the electron gun 5 at the front focal point P1 defined by the front objective lens 2. The beam diverging from the crossover image is made parallel to the optical axis by the front objective lens 2 before it falls on the specimen 1. At this time, the extent of the specimen region irradiated by the parallel electron beam depends on the distance M between a lens aperture plate 9 provided in the second focusing lens 7 and the focal point P1 and also on the diameter d of the aperture formed in the aperture plate 9. The electron beam transmitted through the specimen 1 is focused at the position of the objective lens aperture plate 8 by the rear objective lens 3. The aperture plate 8 is disposed in the back focal plane defined by the lens 3. After passing through the aperture plate 8, the beam is directed into the imaging lens system (not shown), which is so adjusted that the intermediate lens and other elements form an electron microscope image. Thus, an electron microscope image due to the transmitted beam is formed on a fluorescent plate (not shown).
In the TEM mode described above, the specimen image on the fluorescent plate is observed, and a location to be analyzed is found in it. Then, a specimen-moving device (not shown) is operated so that the position of the specimen may be moved relative to the electron beam. The specimen-moving device is adjusted until the location of the image to be investigated is brought to the center of the fluorescent plate, i.e., the optical axis.
After the specimen is set in this way, the lens system is switched to the analysis mode shown in FIG. 2, in which the focusing lenses cause the electron beam 4 to make a crossover image of the electron gun 5 at a point P2 that is sufficiently remote from the front objective lens 2 above the lens 2 to be regarded as infinitely remote. This crossover image is projected onto the specimen 1 to a reduced scale by the front objective lens 2. The divergence angle .alpha. of the beam is determined by the diameter d of the aperture in the aperture plate 9 in the second focusing lens 7. The electron beam transmitted through the specimen passes through the aperture plate 8 and is guided into the imaging lens system which has been adjusted so as to form a diffraction pattern. Thus, a diffraction pattern is presented on the fluorescent plate. Otherwise, the beam transmitted through the specimen is directed into an energy analyzer to make analysis of the energies of the transmitted beam or to detect the X-rays emitted from the specimen for elemental analysis. It is also possible to provide deflection coils above the objective lenses, the coils being supplied with a scan signal. The surface of the specimen is scanned by the electron beam under the control of the coils. A signal acting to modulate the brightness, or the intensity, or a cathode-ray tube in synchronism with the scanning of the beam is affected by the secondary electrons emitted from the specimen. Hence, the secondary electrons can be detected as the modulating signal. In this case, the instrument is employed as a scanning-type electron microscope.
The prior art instrument is switched between the TEM mode and the analysis mode in the manner described above. The problem with this instrument is that the field of view which can be observed in the TEM mode is quite narrow. This field of view may be broadened either by increasing the diameter d of the aperture in the aperture plate 9 in the third focusing lens, generally the focusing lens at the final stage, or by minimizing the distance M between the lens aperture plate 9 and the front focal point P1 defined by the front objective lens. However, as the divergence angle of the electron beam emitted from the electron gun is increased, the current density is reduced. For the above and other reasons, it is impossible to increase the diameter d of the aperture beyond a certain limit. In an attempt to make the distance M as short as possible, the provision of an additional lens between the second focusing lens and the front objective lens has been proposed as disclosed in U.S. Pat. No. 4,306,149. In this proposed system, the distance M can be made short but the additional auxiliary lens makes use of the yoke itself of the objective lens proper, leading to an undesirable situation. In particular, when the excitation to the auxiliary lens is varied, the intensity of the magnetic field produced above and below the specimen changes. Accordingly, if the instrument is switched between the TEM mode and the analysis mode, the axes will not coincide with each other. Consequently, the fields of view will not precisely correspond to each other.